1. Field of the Invention
The present invention concerns an yttria-based slurry composition, e.g. useful for producing slurries needed for the production of ceramic molds for use in casting reactive metals.
2. Description of Prior Art
Aqueous suspensions of ceramic particles, such as yttrium oxide (yttria), zirconium oxide (zirconia), aluminium oxide (alumina), yttria-alumina-zirconia, and zircon (ZrSiO4) are used industrially to form ceramic articles due to their suitability for use as structural materials at high temperatures. These refractory materials often are also used for casting super alloys and reactive metals. An example of such a reactive metal is titanium. Titanium normally reacts with materials used to form the mould, such as oxides, thereby releasing oxygen and forming oxygen-enriched titanium.
A suspension is a system in which typically solid particles are uniformly dispersed in a liquid such as water. Such suspensions are used as ceramic slurries for different purposes, as mentioned above. Ceramic particles normally are at least partially soluble in water. Furthermore ceramic particles tend to hydrate, forming a bond with water. To what extent and how quickly ceramic particles dissolve or hydrate, varies. Moreover, colloidal particles of ceramic particles may agglomerate in water. The extent to which ceramic particles dissolve, hydrate or agglomerate in water based systems depends on many factors, including the nature of the ceramic powder, the oxidation state of the ceramic particle, the pH, the temperature of the system and the dispersants which are used in a ceramic slurry.
A lot of methods are known in the art to stabilize colloidal suspensions, i.e. preventing the suspensions from agglomerating, while simultaneously reducing the dissolution and hydration rates. For instance, three known mechanisms in stabilization procedure include electrostatic, steric and electrosteric mechanisms. These mechanisms e.g. are reviewed in detail by Cesarano and Aksay “Stability of Aqueous Alpha-Al2O3Suspensions with Poly-(methacrylic acid) Polyelectrolyte”, J. Am. Ceram. Soc. 71, p 250-255 (1988).
For yttria prime slurries currently used in industry, there are two general approaches to overcome the tendency for ions liberated by the relatively rapid dissolution of yttria under lower pH conditions. In one, addition of organic bases such as tetraethylammoniumhydroxide to ceramic slurries, e.g. to an yttria slurry, helps preventing the dissolution of silica at high pH levels which are necessary to keep the slurry stable (US 2008/0119347). Other approaches have been to “alloy” the yttria with other non-reactive oxides in a fusion process which tends to reduce the number of dissolution sites on the yttria particles or to coat the yttria with large adsorbed organic molecules or anorganic compounds to achieve the same result: a stable slurry with a relatively long shelf life.
In EP 1 992 430 an yttria based refactory composition is described, obtainable by mixing particles of yttria-based ceramic material and a fluorine containing dopant, and heating the resulting mixture to effect fluorine doping of said yttria based ceramic material to produce stable slurries with a significant improved shelf life in aqueous binder systems. Fluorine-doping of the binder system is not indicated.
In US. 2008/0119347 there is disclosed a mold system for coating reactive alloys by using novel prime coat slurries to form molds or “shells” as they are otherwise known for casting reactive metal components.
According to U.S. Pat. No. 5,621,601 beside colloidal dispersing it seems to be important to reduce the rate of dissolution and/or hydration of colloidal ceramic suspensions to obtain commercially suitable ceramic slurries.
Ceramic materials normally react with water and either partially dissolve (referred to as dissolution or solvation) or form hydrates. The extent of dissolution or hydration varies among different ceramic materials. As ceramic materials dissolve, the dissolved species may substantially change the ionic strength of the solution and consequently particles may agglomerate. In the case of particle hydration, some ceramic particles form a hydroxide surface layer. However, attack by water also may proceed beyond the surface layer and may advance into the body of particles. As a result, size, morphology and the crystal phase of particles may change.
In Hideaki Hamano et al, Langmuir 2000, 16, 6961-6967 the protection of a Nd2O3 sample from hydration by surface fluoridation carried out by stirring the Nd2O3 sample in an aqueous solution of NH4F is described.
In many commercially important ceramic particles, such as alumina (e.g. Al2O3), zirconia (e.g. ZrO2), and zircon (ZrSiO4) to name a few, the dissolution rate and the extent to which dissolution proceeds is low enough so that it does not seem to interfere with their aqueous commercial use, at least under mild acidic or basic conditions such as from about pH 3 to about pH 11. Furthermore, hydration does not seem to form more than a thin surface layer, at least when the particle size is equal to or larger than one micrometer. However, other commercially important ceramic particles, such as magnesia (MgO), yttria-alumina-zirconia, and yttria (e.g. Y2O3), dissolve in aqueous media to a much larger extent and at faster rates than the ceramic materials discussed above. As a result, aqueous processing of these materials such as magnesia, calcia, yttria, yttria-alumina-zirconia is either difficult or even not practicable.
Many attempts have been made by persons skilled in the art of ceramic processing to reduce the dissolution and hydration of ceramic particles, while simultaneously keeping the ceramic particles dispersed (unagglomerated) in suspensions. For example, in U.S. Pat. No. 4,947,927 it is taught that by adjusting the pH of an yttria slurry to high pH values in excess of pH 11 one can make yttria intrinsically less soluble in water, thereby decreasing its sensitivity to water attack.
It seems that, compared to electrostatic stabilization, electrosteric stabilization provides a better method for simultaneously dispersing colloidal particles in suspension and reducing water attack on the ceramic surface.
Monomers have been used to prevent the agglomeration of alumina suspensions, e.g. according to Graule et al. “Stabilization of Alumina Dispersions with Carboxyclic Acids”, Proceedings of the Second European Ceramic Society Conference (1991).
In U.S. Pat. No. 5,624,604 a method for dispersing and reducing the rate of dissolution and/or hydration of colloidal ceramic suspensions is described by adding a non polymeric hydroxylated organic compound to a ceramic suspension. The ceramic suspension typically comprises a colloidal suspension of a metal oxide wherein the metal of the metal oxide is an alkali metal, alkaline-earth metal or rare-earth metal but preferably is magnesium, calcium or a rare-earth metal.
Other methods for increasing the lifetime of a casting slurry e.g. are described in U.S. Pat. No. 6,390,179 according to which one feature of the invention is processing refractory powders at a first hydration level to produce powders having a second, lower hydration level before the processed materials are used to form casting slurries. Processing according to the disclosed methods results in a substantial increase in the lifetime of a slurry made using such processed materials compared to slurries made using materials not processed as described therein.
In U.S. Pat. No. 5,464,797 an aqueous ceramic slurry having from about 70-weight percent to about 85 weight percent of a fused yttria-zirconia material is described. The weight-percent of zirconia in the fused yttria-zirconia preferably varies from about 1.0 weight percent to about 10 weight percent. The slurries are used to form ceramic mold facecoatings for casting reactive materials. These slurries are less sensitive to pH-fluctuations than slurries made from 100 percent yttria (yttria slurries).
Thus, it is understood that persons skilled in the art of ceramic processing have long searched for, and developed methods to increase the lifetime of casting slurries. Despite the prior inventions directed to this objective, there still is a need for convenient and practical methods for increasing the useful lifetimes of investment casting slurries in particular when using binder systems as e g Ammonium Zirconium Carbonate, Zirconium Acetate, or colloidal silica to process such slurries.
In U.S. Pat. No. 5,827,791 yttria-based slurries for use in producing ceramic molds for use in the investment casting of reactive metals are described, particulary titanium and titanium alloys, where the specific preferred binders amongst colloidal silica are ammonium zirconium carbonate and zirconium acetate.
In U.S. Pat. No. 4,740,246 relatively unreactive mold coatings with titanium and titanium alloys that are prepared from zirconia or yttria sols, or mixtures thereof as a binder for refractory composition, such as zirconium oxide, yttrium oxide and mixtures thereof are disclosed. An example is indicated, according to which a cast-sample was made of a slurry containing yttrium oxide and zirconium acetate as essential parts. This sample is very low in alpha case being less than 0.001 inch (0.0254 mm)
From U.S. Pat. No. 4,057,433 a mold for casting molten reactive metals is known, which has a facing portion comprising finely divided particles of oxyfluorides of metals of Group Ma and a back-up portion comprising finely divided particles of shell mold back-up material.